Method for controlling structure of nano-scale substance, and method for preparing low dimensional quantum structure having nano-scale using the method for controlling structure

ABSTRACT

A method for controlling a structure of a nano-scale substance may include irradiating a mixture of low-dimensional quantum structures having a nano-scale with an electromagnetic wave in an oxygen atmosphere, to thereby selectively oxidize a low-dimensional quantum structure having a density of states resonating with the electromagnetic wave used for the irradiation. The method allows a low-dimensional quantum structure having a specific structure to be selectively removed from the mixture of low-dimensional quantum structures having a nano-scale.

PRIORITY CLAIM

This U.S. nonprovisional application claims priority under 35 U.S.C.§121 as a divisional of U.S. application Ser. No. 10/569,548, filed Feb.27, 2006, which is a national stage application filed under 35 U.S.C.§365 of International Application No. PCT/JP2004/011838, filed Aug. 18,2004, which claims priority under 35 U.S.C. §119 to Japanese PatentApplication No. 2003-307602, filed Aug. 29, 2003, the contents of eachof which is hereby incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a method for controlling a structure ofa nano-scale substance whereby a nano-scale substance such as alow-dimensional quantum structure, which may be a one-dimensionalstructure such as a carbon nanotube, or a zero-dimensional structuresuch as a nanoparticle is selectively controlled. The invention alsorelates to a method for producing a nano-scale low-dimensional quantumstructure using such a structure control method.

BACKGROUND

The development of high-tech materials and new materials has asignificant importance as it forms the basis of industry and science andtechnology in a wide variety of fields such as electronics, environmentenergy, and biotechnology. In recent years, the development ofnano-scale substances has drawn many interests since they possesstotally novel properties and functions not found in bulk substances.

Carbon nanotubes are an example of such a nano-scale substance. Carbonnanotubes have a tube-like structure made out of a graphite sheet. Thereare two types of carbon nanotubes: single-walled nanotubes andmulti-walled nanotubes, depending on whether the tube is single-walledor multi-walled. The electrical properties of the carbon nanotube areunique in the sense that the nanotube can be a metal or a semiconductordepending on its chirality.

Referring to FIG. 2, the following describes chirality of the carbonnanotube. As illustrated in FIG. 2, carbon nanotubes have differentchiralities depending on the way the graphite sheets are wound. Carbonnanotubes of differing chirality have different densities of states(electronic states).

As described above, the chirality of carbon nanotubes varies, and assuch a synthesis of carbon nanotubes produces structures of differingchiralities and differing electronic states.

Thus, if the carbon nanotubes were to be used for industrial,manufacturing, and academic purposes, a carbon nanotube of a specificstructure would be needed depending on use. Accordingly, there is ademand for a method of selectively obtaining carbon nanotubes of thesame structure from different structures of carbon nanotubes.

However, to this date, there has been no method that selectively obtainsor removes carbon nanotubes of a specific structure from carbonnanotubes having different electronic states.

The present invention was made in view of the foregoing problem, and anobject of the present invention is to provide a method for controlling anano-scale low-dimensional quantum structure, whereby a low-dimensionalquantum structure of a specific density of state is selectively oxidizedfrom a mixture of low-dimensional quantum structures. The invention alsoprovides a method for producing a nano-scale low-dimensional quantumstructure using such a structure control method.

SUMMARY

In order to achieve the foregoing objects, the inventors of the presentinvention measured Raman spectra of sample single-walled carbonnanotubes at different wavelengths. It was found as a result that thespectra had peaks at different positions depending on the excitedwavelengths. Based on the assumption that single-walled carbon nanotubeswith different densities of states and therefore different electronicstates would resonate with electromagnetic waves of differentwavelengths, the inventors accomplished the invention by finding thatstructures of the nanotubes can be controlled according to the resonanceof the nanotubes.

In order to achieve the foregoing objects, a structure control methodaccording to the present invention includes irradiating a mixture ofnano-scale low-dimensional quantum structures of differing densities ofstates with an electromagnetic wave in an oxygen atmosphere, so as toselectively oxidize a low-dimensional quantum structure of a density ofstates resonating with the electromagnetic wave.

The structure control method according to the present invention may beadapted so that the mixture is irradiated with the electromagnetic waveso as to remove from the mixture the low-dimensional quantum structureof a density of states resonating with the electromagnetic wave.

With the foregoing structure control method, low-dimensional quantumstructures resonating with the irradiating electromagnetic wave absorbmore electromagnetic wave, and increasing the intensity of theelectromagnetic wave oxidizes the low-dimensional quantum structuresresonating with the electromagnetic wave. Thus, a low-dimensionalquantum structure with a specific electronic state can be selectivelyoxidized from a mixture of low-dimensional quantum structures ofdiffering densities of states. Further, by being oxidized, alow-dimensional quantum structure with a specific density of states canbe selectively removed. Further, a low-dimensional quantum structurewith a desired density of states can be selectively retained in themixture. That is, low-dimensional quantum structures with the sameelectronic state can be selectively obtained from low-dimensionalquantum structures of differing electronic states.

In order to achieve the foregoing objects, a producing method of anano-scale low-dimensional quantum structure according to the presentinvention includes the step of irradiating a mixture of nano-scalelow-dimensional quantum structures of differing densities of states withan electromagnetic wave in an oxygen atmosphere, so as to selectivelyoxidize a low-dimensional quantum structure of a density of statesresonating with the electromagnetic wave and thereby remove a structurewith the density of states resonating with the electromagnetic wave.

With the producing method of a nano-scale low-dimensional quantumstructure, a nano-scale low-dimensional quantum structure can beproduced from a mixture of low-dimensional quantum structures, byremoving a low-dimensional quantum structure with a specific density ofstates.

Further, in order to achieve the foregoing objects, a producing methodof a nano-scale low-dimensional quantum structure according to thepresent invention includes the step of irradiating a mixture ofnano-scale low-dimensional quantum structures of differing densities ofstates with an electromagnetic wave in an oxygen atmosphere, so as toselectively oxidize a low-dimensional quantum structure of a density ofstates resonating with the electromagnetic wave and thereby retain astructure with a density of states not resonating with theelectromagnetic wave.

With the producing method of a nano-scale low-dimensional quantumstructure, a nano-scale low-dimensional quantum structure can beproduced from a mixture of low-dimensional quantum structures, byselectively retaining a low-dimensional quantum structure with a desireddensity of states.

Additional objects, features, and strengths of the present inventionwill be made clear by the description below. Further, the advantages ofthe present invention will be evident from the following explanation inreference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) schematize carbon nanotubes irradiated withelectromagnetic waves of different wavelengths according to oneembodiment of the present invention.

FIG. 2 schematizes a graphite sheet, representing differing chiralitiesof carbon nanotubes.

FIG. 3 represents a relationship between energy and a density of statesof carbon nanotubes.

FIG. 4( a) is a view showing an SEM image of sample single-walled carbonnanotubes, and FIG. 4( b) is a magnified view of FIG. 4( a).

FIG. 5 represents a Raman spectrum of sample single-walled carbonnanotubes in the high frequency range.

FIG. 6 represents Raman spectra of sample single-walled carbon nanotubesirradiated with laser beams of different wavelengths.

FIGS. 7( a) through 7(c) represent Raman spectra of sample single-walledcarbon nanotubes before and after 30 minute irradiation of a laser beamat 20 kW/cm².

FIGS. 8( a) through 8(c) represent Raman spectra of sample single-walledcarbon nanotubes before and after 2 hour irradiation of a laser beam at10 kW/cm².

FIG. 9 is a graph representing changes in relative intensity of peaks inthe Raman spectra shown in FIGS. 7( a) through 7(c) and FIGS. 8( a)through 8(c).

FIG. 10 represents Raman spectra of sample single-walled carbonnanotubes before and after 70 minute irradiation of a laser beam at 50kW/cm².

DETAILED DESCRIPTION

Referring to the attached drawings, the following will describe oneembodiment of the present invention. It should be noted that theinvention is not limited by the following description.

Preferably, a nano-scale structure subjected to a structure controlmethod of the present invention is a low-dimensional quantum structure.As used herein, the “low-dimensional quantum structure” refers to azero-dimensional structure (sphere) such as nanoparticles or other ultrafine particles, and a one-dimensional structure (stylus) such asnanotubes and nanowires. Further, as used herein, the term “nano-scale”refers to structures with a particle size or outer diameter of not morethan 100 nm. However, a structure control method of the presentinvention can suitably be used for those with a particle size or outerdiameter of not more than 10 nm, and more suitably for those with aparticle size or outer diameter of not more than 3 nm.

It is preferable that the low-density quantum structure have a spikeddensity of states. An example of such a structure is a nanotube. Thenanotube may be single-walled or multi-walled, but a single-walledstructure is more preferable. The nanotube has an outer diameter ofpreferably not more than 10 nm, or more preferably not more than 3 nm.As used herein, “spiked” means that the peak of the density of stateshas a sharp end, instead of a step-like end or a radiating end.

In the case where a single-walled carbon nanotube is used by a structurecontrol method of the present invention, the single-walled carbonnanotube can be formed by ordinary methods. For example, an arkdischarge method, a laser evaporation method, or a chemical vapordeposition method (CVD) may be used with a catalyst metal, which may be,for example, iron, nickel, cobalt, platinum, palladium, rhodium,lanthanum, or yttrium. In the case where the CVD method is used, carbonnanotubes can be formed on a substrate by the high-temperature reactionof acetylene, benzene, ethane, ethylene, ethanol, or the like with acatalyst metal. The material of the substrate is not particularlylimited as long as it can withstand high temperature. For example,silicon, zeolite, quarts, and sapphire can be used.

In a structure control method of the present invention, theelectromagnetic wave used to irradiate the low-dimensional quantumstructure is not particularly limited as long as it can resonate thelow-dimensional quantum structure to be oxidized and is strong enough tooxidize the low-dimensional quantum structure. A non-limiting example isa laser beam. Further, with use of an electromagnetic wave of a widewavelength range for example, low-dimensional quantum structures withdifferent electronic states can be oxidized at once. Note that, theintensity of the electromagnetic wave may be measured by measuringenergy density for example.

Further, in a structure control method of the present invention, theelectromagnetic wave used to irradiate the low-dimensional quantumstructure may be converged. Converging the electromagnetic wave allowsfor localized irradiation of a mixture of low-dimensional quantumstructures. More specifically, low-dimensional quantum structures to beused for different purposes can be selectively oxidized and removed atdifferent locations. The electromagnetic wave can be converged by anordinary method, using a lens for example.

Referring to FIGS. 1( a) and 1(b), the following will describe astructure control method according to the present invention. A structurecontrol method of the present invention is performed in an atmosphere ofoxygen, in order to oxidize a low-dimensional quantum structure of aspecific structure. For example, the method can be performed in anatmosphere. As shown in FIGS. 1( a) and 1(b), with the irradiation ofelectromagnetic waves of different wavelengths in an atmosphere, thelow-dimensional quantum structures resonating with the electromagneticwaves in the mixture (blanked in FIG. 1( a), hatched in FIG. 1( b))absorb more electromagnetic wave. Here, if intensity of theelectromagnetic waves is increased, only the resonating low-dimensionalquantum structures are oxidized and these low-dimensional quantumstructures cannot retain their original structures. In the case wherethe low-dimensional quantum structures are carbon for example, thelow-dimensional quantum structures resonating with the irradiatingelectromagnetic waves are converted into COx by being oxidized andtherefore can be removed. Note that, low-dimensional quantum structuresnot resonating with the electromagnetic waves are not oxidized andremain.

In the following, description is made as to how resonance occurs. Carbonnanotubes of differing chirality have different densities of states. Asshown in FIG. 3, when a single-walled carbon nanotube with certainchirality (as represented by the density of states in FIG. 3) isirradiated with an electromagnetic wave of a certain wavelength, thelow-density quantum structure resonates and absorbs more electromagneticwave when the energy difference between spikes is close to the energy ofthe electromagnetic wave. Note that, the energy difference betweenspikes in the density of states is different when the chirality isdifferent.

As described above, with a structure control method according to thepresent invention, a low-dimensional quantum structure with a specificdensity of states can be selectively oxidized and eliminated from amixture of low-dimensional quantum structures having different densitiesof states. Further, by oxidizing different kinds of low-dimensionalquantum structures, a low-dimensional quantum structure with a desireddensity of states can be selectively retained in the mixture. That is,low-dimensional quantum structures of the same density of states can beselectively obtained from low-dimensional quantum structures havingdifferent densities of states.

Whether the low-dimensional quantum structures resonating with theirradiating electromagnetic waves have been oxidized, or whethernon-resonating low-dimensional quantum structures were not oxidized canbe found by measuring the spectrum of the low-dimensional quantumstructures before and after the irradiation of the electromagneticwaves, using Raman spectrometry for example. More specifically, whetheror not low-dimension quantum structures have been oxidized can beconfirmed by measuring the Raman spectrum before and after theirradiation of electromagnetic waves of different wavelengths, and thenmeasuring a reduction in the peak intensity of the spectrum. Here, inorder to prevent non-target low-dimensional quantum structures frombeing oxidized, the spectrum needs to be measured with electromagneticwaves of low energy density. The method of confirming oxidation is notjust limited to the foregoing method.

EXAMPLES

The following will describe Examples of the present invention in detailbased on Experiment 1 through Experiment 3. It should be noted here thatthe invention is not limited by the following description.

Experiment 1 Single-Walled Carbon Nanotube

A sample single-walled carbon nanotube was synthesized with ethanol thathas been applied on a silicon (Si) substrate coated with aniron-containing catalyst. The reaction was performed at 900° C. using athermal CVD method.

The sample single-walled carbon nanotube prepared in the experiment wasobserved under SEM. FIGS. 4( a) and 4(b) are resulting SEM images. Asshown in FIG. 4( a), a growth of SWNT was confirmed on the substrate.FIG. 4( b) is a magnified view of FIG. 4( a).

A Raman spectrum of the sample was also measured. FIG. 5 represents theresult. As the excited light source, an Ar ion laser (λ=514.5 nm) wasused.

As shown in FIG. 5, the spectrum in the high frequency range had twolarge peaks, called G band and D band. The G band in the vicinity of1590 cm⁻¹ originates from graphite (or more accurately, oscillation inthe hexagonal lattice of the carbon atoms). The D band in the vicinityof 1350 cm⁻¹ originates from defects in the single-walled carbonnanotube, or carbon atoms with dangling bonds, such as amorphous carbon.As such, larger values of G/D (G band/D band) intensity ratio provideSWNT of better crystallinity. The sample prepared in this experiment hada G/D ratio of about 50, providing high-quality SWNT.

Experiment 2 Excited Wavelength Dependency of Raman Spectra

The sample single-walled carbon nanotube obtained in Experiment 1 wasirradiated with laser beams (energy density of 1 kW/cm², wavelengths of514.5 nm, 488.0 nm, and 457.9 nm) in an atmosphere, and Raman spectrawere measured. As the light source, an Ar laser was used. FIG. 6 showsthe results. The Raman spectra shown in FIG. 6 respectively correspondsto, from the top, the wavelengths of 457.9 nm, 488.0 nm, and 514.5 nm ofthe irradiating laser beams. As can be seen from FIG. 6, the Ramanspectra of different wavelengths had peaks at different positions. Thisindicates that single-walled carbon nanotubes with different densitiesof states are resonating with the different wavelengths of theirradiating laser beams.

Experiment 3 Raman Spectra after Irradiation of High Energy DensityLaser Beam

The sample carbon nanotube obtained in Experiment 1 was irradiated withan Ar laser (energy density of 20 kW/cm², wavelength of 514.5 nm) for 30minutes in an atmosphere, and Raman spectra were measured. Themeasurement of Raman spectra was performed according to the procedure ofExperiment 2. FIGS. 7( a) through 7(c) show the results. Note that, ineach of FIGS. 7( a) through 7(c), the upper spectrum is beforeirradiation of the laser beam, and the lower spectrum is afterirradiation of the laser beam. The same also applies to FIGS. 8( a)through 8(c), and FIG. 10. As can be seen from FIGS. 7( a) through 7(c),after irradiation of the laser beam with the energy density of 20kW/cm², there was a slight decrease in the peak intensity of thesingle-walled carbon nanotube of a density of state resonating with thelaser beam of each different wavelength.

In the same manner, the sample carbon nanotube was irradiated with an Arlaser for 2 hours at the energy density of 10 kW/cm², and Raman spectrawere measured. The results are shown in FIGS. 8( a) through 8(c). As canbe seen from FIGS. 8( a) through 8(c), after irradiation of the laserbeam with the energy density of 10 kW/cm², there was a significantdecrease in the peak intensity of the single-walled carbon nanotube of adensity of state resonating with the laser beam of a 514.5 nmwavelength. As for the carbon nanotubes of densities of statesresonating with the laser beams of 488.0 nm and 457.9 nm wavelengths,there was no large decrease in the peak intensity even after theirradiation of the laser beams, as shown in FIGS. 8( b) and 8(c).

FIG. 9 is a graph representing the average intensity ratio of the peaksmeasured before and after irradiation of the laser beams. As can be seenfrom FIG. 9, irradiation of the laser beam of a 514.5 nm wavelength atthe energy density of 10 kW/cm² selectively oxidized single-walledcarbon nanotubes resonating with the 514.5 nm wavelength. That is, byadjusting the wavelength and energy density of irradiating light,single-walled carbon nanotubes of a density of states resonating withthe wavelength of the irradiating light were selectively oxidized andtherefore selectively removed. Carbon nanotubes of a density of statesnot resonating with the wavelength of irradiating light were notoxidized and remained. By being oxidized, the carbon nanotube was lostin the form of CO.

In the same manner, the sample carbon nanotube was irradiated with an Arlaser for 70 minutes at the energy density of 50 kW/cm², and a Ramanspectrum was measured. FIG. 10 shows the results. As shown in FIG. 10,the Raman spectrum from the Si substrate only had a peak afterirradiation of an Ar laser at the energy density of 50 kW/cm². Themeasurement therefore showed that irradiation of an Ar laser for 70minutes at the energy density of 50 kW/cm² oxidized and eliminated mostof the single-walled carbon nanotubes.

As described above, a structure control method according to the presentinvention includes irradiating a mixture of nano-scale low-dimensionalquantum structures of differing densities of states with anelectromagnetic wave in an oxygen atmosphere, so as to selectivelyoxidize a low-dimensional quantum structure of a density of statesresonating with the electromagnetic wave.

The mixture may be irradiated with the electromagnetic wave so as toremove therefrom a low-dimensional quantum structure of a density ofstates resonating with the electromagnetic wave.

Further, a producing method of a nano-scale low-dimensional quantumstructure according to the present invention includes the step ofirradiating a mixture of nano-scale low-dimensional quantum structuresof differing densities of states with an electromagnetic wave in anoxygen atmosphere, so as to selectively oxidize a low-dimensionalquantum structure of a density of states resonating with theelectromagnetic wave and thereby remove a structure with the density ofstates resonating with the electromagnetic wave.

Further, a producing method of a nano-scale low-dimensional quantumstructure according to the present invention includes the step ofirradiating a mixture of nano-scale low-dimensional quantum structuresof differing densities of states with an electromagnetic wave in anoxygen atmosphere, so as to selectively oxidize a low-dimensionalquantum structure of a density of states resonating with theelectromagnetic wave and thereby retain a structure with a density ofstates not resonating with the electromagnetic wave.

The low-dimensional quantum structures may be nanotubes ornanoparticles.

When the low-dimensional quantum structures are nanotubes ornanoparticles, the density of states has a spiked structure. Thus, withthe foregoing structure control method, the low-dimensional quantumstructures can desirably resonate with an electromagnetic wave of aspecific wavelength.

The low-dimensional quantum structures may be carbon or boron nitride.

Some types of carbon or boron nitride have well defined nano-scalestructures. Thus, a structure control method according to the presentinvention can directly be used for industrial, manufacturing, andacademic purposes.

Further, the low-dimensional quantum structures may have a single-walledstructure.

Single-walled low-dimensional quantum structures have a specific densityof states. Thus, in using the structure control method, a wavelength ofthe electromagnetic wave used to resonate a specific low-dimensionalquantum structure can be selected more easily.

Further, the electromagnetic wave may be a laser beam.

With the use of a laser beam as the electromagnetic wave, the wavelengthor intensity of the electromagnetic wave used for irradiation can beadjusted more easily. Thus, in using the structure control method, amixture of low-dimensional quantum structures can be efficientlyirradiated with a high-energy electromagnetic wave, enabling alow-dimensional quantum structure of a specific density of states to beoxidized and removed. Since the laser beam is a highly linear beam oflight and does not spread easily, it can be converged easily. Theelectromagnetic wave is converged for the following reason. Convergingthe electromagnetic wave allows for localized irradiation of a mixtureof low-dimensional quantum structures. More specifically,low-dimensional quantum structures used for different purposes can beselectively oxidized and removed at different locations.

The embodiments and concrete examples of implementation discussed in theforegoing detailed explanation serve solely to illustrate the technicaldetails of the present invention, which should not be narrowlyinterpreted within the limits of such embodiments and concrete examples,but rather may be applied in many variations within the spirit of thepresent invention, provided such variations do not exceed the scope ofthe patent claims set forth below.

INDUSTRIAL APPLICABILITY

According to the present invention, there are provided a structurecontrol method, and a method for producing a nano-scale low-dimensionalquantum structure using the structure control method. With the methodsof the present invention, a low-dimensional quantum structure of aspecific density of states resonating with the wavelength of theelectromagnetic wave used for irradiation can be selectively oxidized ina mixture of low-dimensional quantum structures. By being oxidized, thelow-dimensional quantum structure with a specific density of states canbe selectively removed from the mixture. Further, a low-dimensionalquantum structure with a desired density of states can be selectivelyretained in the mixture.

The present invention is therefore applicable to a wide variety offields using nano-technology, including, for example, electronics,information communications, chemistry, materials, environment, energy,and many areas of life science, such as biotechnology, medicine, andpharmaceuticals. For example, the invention has many uses in thestructure control of functional and structural materials used foroptical devices, electronic devices, and micro devices. The invention isparticularly effective in the structure control of functional materialsused for electron-emissive materials, probes such as STM, thin lines formicro machines, thin lines for quantum effect elements, field effecttransistors, single-electron transistors, hydrogen absorbing materials,and bio-devices.

1. A producing method of a nano-scale low-dimensional quantum structure,the method comprising: irradiating a mixture of nano-scalelow-dimensional quantum structures of differing densities of states withan electromagnetic wave in an oxygen atmosphere, so as to selectivelyoxidize a low-dimensional quantum structure of a density of statesresonating with the electromagnetic wave and thereby retain a structurewith a density of states not resonating with the electromagnetic wave.